Fluorescent lamps are used in a variety of environments ranging from residential and commercial buildings to aircraft. (While this invention was developed for use in aircraft, and is described in connection with such use, it is to be understood that the invention can also be used in other environments.) Modern commercial aircraft have begun using fluorescent lamps in cockpit aircraft instrument panel floodlighting systems and in aircraft cabins in order to obtain the constant, more appealing color temperature and uniform diffuse light generated by flurrescent lamps, and to avoid the heat generated by incandescent lamps, which is difficult to dissipate. In order for fluorescent lamps to be usable in aircraft instrument panel floodlighting systems, they must be able to offer a range of illumination levels. Under bright sunlight conditions, instrument panel floodlighting must have the ability to fill in shadows. Under nighttime flying conditions, instrument panel floodlighting must be low enough to not distract a pilot from seeing nighttime features with fully dark adapted eyes. Thus, in order for fluorescent lamps to be usable in aircraft instrument panel floodlighting systems, they must be dimmable. The use of fluorescent light in cabin illumination systems on aircraft designed for long flights is highly desirable because the effect of the soft light provided by fluorescent lamps provides a more comfortable environment. It is also desirable to dim, albeit at much lower ratios, aircraft cabin illumination systems so that lighting can be reduced for sleeping. Because weight and size are at a premium onboard an aircraft, it is desirable that the control system, or ballast, for controlling aircraft fluorescent lamps be lightweight and compact.
One way of creating a dimmable fluorescent lamp is to use an electronic ballast to control the starting and operation of the lamp. Electronic ballasts, whether dimmable or not, are particularly desirable in an aircraft environment because electronic ballasts, as opposed to other types of ballasts, such as non-switching iron core magnetic ballasts, are usually small, lightweight and highly efficient, and easy to control with low power signals. Unfortunately, in the past, electronic ballasts have had a number of features that have made them less desirable for use in aircraft instrument panel floodlighting systems as well as in other environments.
The conventional approach to provide an electronic ballast is to convert input AC to DC (if the input voltage is not DC) and, then, convert DC to high frequency AC. The high frequency AC is used to power the fluorescent lamp or lamps. Typically, the AC frequency is above 10 kHz because operating fluorescent lamps at frequencies greater than 10 kHz increases the lumens per watt (e.g., the efficiency) of fluorescent lamps. Weight advantages are realized simply because the weight of high frequency transformers and inductors is less than the weight of low frequency transformers and inductors.
In conventional high frequency ballasts, a voltage sufficiently large to start the lamp is shaped into a sine wave by an L-C tank circuit. Current limiting is provided by an inductive device similar to the inductive devices used in lower frequency (e.g., 400 Hz) ballasts. A major disadvantage of this ballast circuitry is that it provides poor magnetic volt-ampere utilization.
Another disadvantage of conventional high frequency electronic ballasts, as well as other ballasts, is the need to store energy to keep the lamp lit during the valley between rectified sine wave half cycles. Typically, this is done by placing a hold-up capacitor on the DC side of the input rectifier. The use of a hold-up capacitor introduces several problems and trade offs. If the capacitor is large, the input conduction angle becomes small, resulting in poor power factor, large peak input current, and huge turn-on surge currents. Large current flows require the inclusion of expensive components designed to handle the contemplated current without breakdown. Contrariwise, if the hold-up capacitor is relatively small, the magnetic portion of the circuit must be increased to provide still more boost and drop. Further, the lamp crest factor is increased by the resultant increase in the ripple of the input voltage.
Because most conventional high frequency electronic ballasts operate open loop, line amplitude variations produce lamp intensity variations. Further, because conventional high frequency electronic ballasts use high frequency starting and operating power, they generate an undesirable amount of electromagnetic interference (EMI).
The present invention is directed to providing an electronic ballast for gas discharge tubes, e.g., fluorescent lamps, that overcomes the disadvantages of prior art high frequency, electronic lamp ballasts.